Conventionally, to restore an image that is deteriorated in image quality it is necessary to analyze the cause of the deterioration, determine a deterioration model function, and apply its inverse function to the deteriorated image. Various causes of deteriorations are possible, such as a uniform movement of a camera (imaging device such as a video camera) and blurring caused by the optical system of a camera. Therefore, in restoring an image, different model functions may be used for respective causes of deteriorations. Unless the cause of deterioration is found, it is difficult to restore a deteriorated image because a model function cannot be determined.
In addition, it is frequently the case that even if a model function of a deterioration is established, there is no inverse function for restoration that corresponds to the model function. In such a case, it is difficult to perform evaluation for determining the optimum model.
Conventionally, error recovery has been achieved by correlation evaluation. For example, some recovery choices have been implemented using a conventional error pixel recovery method. FIG. 1A shows a conventional error recovery block diagram. Using neighboring data, which are shown in FIG. 1B, spatial inclinations of the target data are detected. In this example, the inclinations regarding four directions are evaluated according to the formulae which are shown in FIG. 1C. An interpolation filter is chosen where the inclination value, E.sub.i, is the smallest among four values. In addition to the spatial inclination, a motion factor is also evaluated for error recovery. In the case of the motion area, a selected spatial filter is used for error recovery. On the other hand, the previous frame data at the same location as the target data are used for error recovery. This evaluation is performed in the evaluation block of FIG. 1A.
The conventional error recovery process shown in FIGS. 1A-1C may cause many serious degradations on changing data, especially on object edges. Actual signal distribution typically varies widely, so these problems are likely to occur. Therefore, there is a need for a way to restore a deteriorated signal to an undeteriorated signal which minimizes degradations on changing data.